1. Field of the Invention
The present invention relates to a semiconductor apparatus having a thin semiconductor film attached to a substrate, and more particularly to the provision of probe pads in the apparatus. The invention also relates to a printing head and printer using the invented semiconductor apparatus.
2. Description of the Related Art
An example of a printer in which the present invention can be practiced to advantage is an electrophotographic printer employing light-emitting diodes as light sources. A conventional electrophotographic printer of this type has a light-emitting diode (LED) print head comprising a linear array of LEDs paralleled by driving circuits that supply current to the LEDs. The LEDs are formed in one or more compound semiconductor chips comprising, for example, gallium arsenide (GaAs); the driving circuits are integrated into one or more silicon (Si) semiconductor chips. The LED array chips and the driver integrated circuit (IC) chips are interconnected by wire bonding, and have pads for the attachment of bonding wires.
The wire bonding pads are in general larger than the LEDs. If a separate wire bonding pad is provided for each LED, the layout of the wire bonding pads presents a problem to which various solutions have been adopted. One solution, for example, employs a staggered arrangement of wire bonding pads and offsets the wiring that leads from the wire bonding pads to the LEDs, as described in Japanese Unexamined Patent Application Publication No. 10-150221.
Regardless of how the wire bonding pads are laid out, they take up considerable space on each LED array chip. From the standpoint of effective use of expensive compound semiconductor materials, this is highly uneconomical, and it prevents the cost of the LED array chips from being reduced beyond a certain point. The wire bonding pads on the driver chips are similarly uneconomical.
Furthermore, although an LED array chip is typically about three hundred micrometers (300 μm) thick, the LEDs occupy only about the uppermost five micrometers (5 μm) of the chip thickness, the rest of the thickness being necessary merely for mechanical support. This also is highly uneconomical.
An attractive solution to this problem is to form the LEDs in a thin compound semiconductor film, which is then attached to the silicon substrate of the driver chips, and to connect the LEDs to their driving circuits through conductive lines formed on the chip surface, instead of by wire bonding. Considerable compound semiconductor material can thus be saved, since none has to be used for mechanical support, or to provide space for wire bonding pads. Testing of the LEDs, the driving circuits, and their interconnections presents a problem, however.
In particular, if the only way to test the driving circuits is to measure the light output from the LEDs, a defective driving circuit in the silicon substrate will remain undiscovered until the thin compound semiconductor film has been attached; then when the defect is found, both the silicon substrate and the attached compound semiconductor film will have to be discarded, even though there may be no defects in the compound semiconductor film. It would be desirable if silicon substrates with defective circuits could be screened out before any compound semiconductor films were attached.
More generally, it would be desirable to improve the testability of any semiconductor apparatus of the type having a thin semiconductor film attached to a substrate, so that defects could be found as early as possible in the fabrication process, to avoid wasting further time and materials on defective devices.